Studies in vitro of the binding of repressor from lambda cIts mutants to lambda DNA will be continued. We will also attempt to demonstrate binding activity attributable to the product of gene tof. Data in press indicate that lambda N negative cI converted lysogens oontain 7-8 prophages integrated in tandem, which replicate at a higher rate than the host chromosomal DNA. Direct evidence for this structure will be sought using radioautography and electron microscopy. Lysogens containing a low number of lambda N negative cItsA prophages die when heated, while lysogens with 7-8 prophages of the same genotype survive. In both types of lysogen, studies will be made, after derepression, of lambda mRNA, lambda DNA, and tof product.